chapter 20

• discontinuous characteristics

  limited number of traits, no intermediate forms
• continuous characteristics

  range or gradation of characteristics
• discontinuous characteristics are controlled by a few genes with multiple alleles
• continuous characteristics are controlled by many genes (polygenic)
• environmental factors have no effect on discontinuous characteristics
• environmental factors have a significant impact on continuous characteristics
• allele

  version of a gene that has differences in the DNA base sequences that is expressed as slightly different polypeptide
• gene

  length of DNA that codes for one of more polypeptide
• locus

  position of a gene or chromosome
• chromosome

  DNA tightly coiled many times around proteins called histones that support its structure
• genotype

  genetic make up of an organism in terms of the alleles it contains for a particular characteristic
• phenotype

  characteristics expressed in an organism
• steps for a genetic diagram

  1. phenotype of both parents
  2. genotype of both parents
  3. gametes of both parents
  4. draw punnet square
  5. state proportion of each genotype
  6. state corresponding phenotype

  
• codominant

  both alleles expressed in the phenotype of a heterozygote
• linkage

  two or more genes present on the same chromosome that are normally inherited together, e.g freckle and red hair are on the same gene
• sex linkage

  gene that codes for the characteristics found on one of the sex chromosomes
• alleles are codominant if they are both expressed in the phenotype of a heterozygote, they can be represented by 2 capital letters superscript to the letter representing the gene
• in mammals females are homogametic XX
• in mammals males are heterogametic XY
• in birds, butterflies and moths, females are heterogametic (XY) and males are homogametic (XX), opposite way round to other mammals
• haemophilia

  caused by a recessive allele that encodes non functioning factor VIII, the gene for factor VIII is located on the X chromosome, hemophilia is sex linked
• F1 cross

  2 parents who are both homozygous with opposite alleles are bred together, one has dominant alleles for both gene loci and the other has recessive alleles for both gene loci
• F2 cross

  2 heterozygous parents are bred together, the offspring of the F1 cross are used
• ratio for F2

  9:3:3:1
• mutation

  formation of new alleles leads to genetic variation
• sexual selection

  increasing frequency in alleles resulting in characteristics improving mating success
• gene flow

  immigration and emigration result in movement of alleles between populations
• genetic drift

  change in allele frequency due to random nature of mutation in a small population, a new allele will have a greater impact in a smaller population than in a much larger population where there is a greater number of alleles in the gene pool
• natural selection

  individuals with characteristics that improve their survival rate and therefore reproductive rate will increase in number and so will the alleles coding for the characteristics
• why is a large gene pool beneficial

  • more genetic diversity
  • more phenotypic variation
  • more resistance to environmental change such as new diseases, natural disasters, environmental changes ect
  • more chance of a population being able to adapt to local changes
  • less chance of recessive genetic conditions as less chance of being closely related in the group size
• small population can often have limited genetic diversity due to the small number of individuals present, some large populations may have limited genetic diversity too, e.g when a population that was small undergoes rapid expansion
• factors affecting population size

  1. density dependent factors are dependent on population size and include competition, predation, parasitism and communicable diseases
  2. density independent factors affect populations of all sizes in the same way including climate change, natural disasters, seasonal change and human activities
• types of selection

  selection causes some traits to survive and spread while others are lost, a selection pressure determines which traits are successful, selection can be represented using graphs showing the distribution of individuals with a particular trait, there are 3 types:

  1. disruptive
  2. stabilising
  3. directional
• a starting population has a normal distribution of traits

  
• factors in a habitat that limit survival offer environmental resistance to population growth e.g limit population size, only the best adapted organisms will survive and breed, this is natural selection
• disruptive selection

  selection pressure towards the extremes to make 2 modal values

  
• stabilising selection

  selection pressure towards the centre increases the number of individuals at the modal value

  
• directional selection

  selection pressures towards one extreme moves the mode in this directional although examples of this in nature are rare

  
• selection is not only the important aspect of evolution, chance also plays a major role, true in small populations, random processes have a small effect in a large population but can have a big impact on genetic variation in a small population as there are less individuals
• chance events include mutations producing new alleles and new phenotypes, these chance changes in a population's gene pool is known as genetic drift